1. Field of the Invention
This invention relates to subnanometer catalysts and methods for fabricating subnanometer and nanometer size-selected catalyst clusters, and more specifically, this invention relates to catalyst clusters comprising 4 to 20 atoms of catalytic metal and methods of producing the clusters on a support substrate.
2. Background of the Invention
Alkanes are typical feedstocks for transformation to alkenes, aromatics and chemicals containing value added moieties. Dehydrogenation is a route to such transformations but dehydrogenation is an endothermic process requiring significant energy input.
Oxidative dehydrogenation (ODH) of propane to propylene is a multibillion dollar industrial process. Two classes of catalysts are used: VOx and Pt-based. The vanadia based ones are highly selective but their activity is relatively low. Pt-based catalysts are more active but their selectivity is low. U.S. Pat. No. 6,781,018 issued to Liu et al. on Aug. 24, 2004 discloses the use of molybdenum and/or vanadium oxides for converting dimethyl ether to formaldehyde.
ODH of alkanes is exothermic, and thus an attractive alternative to dehydrogenation. However, current ODH catalysts have limited activity and/or poor selectivity resulting from inability to prevent complete oxidation. For example, FIG. 1 depicts a preferable reaction pathway “A” for propane undergoing oxidative dehydrogenation versus just as likely non-preferable reaction pathways “B”, “C”, “D”, and “E”.
Silberova et al. Appl. Catal. A: General 276 17-28 (2004) discusses oxidative dehydrogenation of ethane and propane at short contact times.
U.S. Pat. No. 5,623,090 issued to Haruta et al on Apr. 22, 1997 discloses catalysts comprising gold particles deposited on titanium oxide carrier. These catalysts were used to produce alcohol, ketone, and epoxides via oxidation of hydrocarbons.
U.S. Pat. No. 6,252,095 issued to Hayashi et al on Jun. 26, 2001 also utilizes gold particles to partially oxidize unsaturated hydrocarbons.
The configuration of catalyst support surfaces also has been investigated. U.S. Pat. No. 7,402,719 issued to Brophy et al on Jul. 22, 2008, discloses the use of 1-2 millimeter diameter channels containing catalysts to facilitate catalytic dehydrogenation. However, the short contact times embodied by Brophy results in relatively low product selectivities, for example 18 percent in the case of propylene formation.
U.S. Patent Publication Number 2008/0039315 by Ma et al, published on Feb. 14, 2008, discloses the use of nanotubes containing catalysts. A drawback to this configuration is a lack of control as to size of catalyst clusters and reaction sites, inasmuch as wet chemistry is utilized to combine the support substrate with catalyst material. This results in different size distributions, and poorer selectivity. Also, the less than 5 nm pore sizes of Ma can result in clogging of reaction-ways.
A need exists in the art for catalysts and a method for supporting catalysts which exhibit high reactivity and selectivities of greater than 50 percent. The catalysts should not require a constant input of energy to operate. Also, the catalysts should operate on selective moieties in a chemical system so as to optimize conversion rates without the generation of unwanted products. Finally, the catalysts should exhibit superior reactivity and selectivities at lower temperatures than typical catalyst operating temperatures.